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We will download some public domain raster data for Brazil and view it in QGIS. Next, we
will merge these into a single mosaic and clip it using a country boundary to
get a single seamless dataset for the country.

Browse to the directory with the individual images. Hold down the Ctrl
key and click on the image files to make a multiple selection. Click
Open.

You will see the images load up in the Table of Content on the left panel.
Now let us create a single Mosaic image from all these individual images.
Click on Raster ‣ Miscellaneous ‣ Merge.

Note

The Raster menu in QGIS comes from a core plugin called
GdalTools. If you do not see the Raster menu,
enable the GdalTools plugin from Plugins ‣
Manage and install plugins ‣ Installed. See Using Plugins for more details.

In the Merge dialog, click Select... next to
Input files and browse to the directory containing all
the individual geotiffs. Keep holding Ctrl key and select all the.
subsets. Now click Select... next to Output file
and name the output as Brazil_mosaic.tif. At the bottom, check the box next to
Load into canvas when finished. Click OK.

You will get a pop-up message saying Processing complete, once the mosaic is
created and loaded to the QGIS Canvas. You will see that the individual
images and now combined and mosaiced into a single layer. You can now turn
off individual layers by un-checking the box next to them.

Another Raster operation you can do is to subset or crop an image. We can
use a polygon from a vector layer to crop the raster to the exact shape.
Let’s load the country polygons shapefile we downloaded from Natural Earth.
Go to Layer ‣ Add Vector Layer.

Select the ne_10m_admin_0_countries.zip file and click
Open. When prompted to select the layer within the zip file,
select ne_10m_admin_0_countries.shp.

Once the vector layer is loaded, we want to select and extract the polygon
for Brazil. Select the Select Single Feature tool from the
toolbar.

Name your output as brazil_boundary.shp and make sure Add
saved file to map box is checked. click OK.

You will see the Brazil boundary polygon now loaded in QGIS.

Now go to Raster ‣ Extraction ‣ Clipper.

Select the input file (raster) as Brazil_mosaic. Name the
Output file as Brazil_mosaic_clipped. In the
Clipping mode section, choose Mask layer. Select
the newly created brazil_boundary as the mask layer. Check the box next
to Load into canvas when finished. Click OK.

The new cropped layer will be loaded into QGIS. You will notice the black
pixels surrounding the actual mosaic. Let’s remove that. Right-click on the
Brazil_mosaic_clipped layer and select Properties.

Go to the Transparency tab, and add 0 as an
Additional no data value.

Now you have a nice mosaic cropped to a country boundary that you can use
in your project as a background layer or do further analysis on.